This one I got from Jeff. Sharp criticism... The text written in italic is my original text, followed with the comment.

gcc (GNU C Compiler) is actually a collection of frontend tools that

Actually, gcc == GNU Compiler Collection - the whole family is
referred to as gcc.

Preprocessing: Producing code that no longer contains directives. Things like "#if" cannot be understood directly by the compiler, so this must be translated into real code. Macros are also expanded at this stage, making the resulting code larger than the original.

It also pulls in headers.

..manipulate them further. This work is done in multipass style, which demonstrates that it sometimes takes more than one scan through the source code to optimize.

It doesn't scan the source - it scans the intermediate format, which used to be RTL, but which is something else now.

...As you may already be aware, registers can be accessed hundreds or thousands times faster than RAM cells.

Exaggeration - Maybe ~100 cycles for going out to main memory, but these things will be in cache, so might cost a few cycles.

0x7530 is 30,000 in decimal form, so we can quickly guess the loop is..

0x7530 is hex, "0x7530 is 30,000 in hexadecimal form" or "0x7530 in decimal is 30,000"

simplified. This code represents the innermost loop and the outermost loop ("for(j=0;j<5000;j++) ... for(k=0;k<4;k++)") because that is literally a request to do 30,000 loops. Note that you just need to...</i>

5000 * 4 = 20000 loops.

Author's note: I admit this is solely my own mistake that confused number of loops with the current value of accumulator (acc variable). The correct sentence should be "this code represents the middle and the innermost loop (for(j=0;j<5000;j++) ...for(k=0;k<4;k++)</code>). In the end of these loops, accumulator is increased by 30,000".

To illustrate them better, here are the codes with inline comments. First check #1, then #2 and so on to understand the flow.